Many conventional containers have a standard snap cap and neck finish; most aspirin bottles utilize this type of container. In this basic snap cap design, the extended skirt of the cap secures under a protrusion on the neck of the vial such that there is one point of contact between the skirt and vial upon sealing the container. In addition, those designs which have more than one point of contact do not generally have tight dimensional tolerances between the cap and container contact points. This type of cap can only be used on vials which have a snap ring for engagement with the skirt of the snap cap.
The conventional design of the snap cap does not provide for ease of assembling the cap and the vial or for ease of removing the cap from the vial. The snap cap requires the use of downward pressure to apply the cap and upward pressure to remove the cap. Such pressure typically is exerted by the thumb of the user. Advantageously, a snap is heard or felt when the cap is positioned and the container is sealed. No tools are required either to apply or to remove the cap.
The cap and container are typically made of plastic. This is advantageous because metal is undesirable in laboratory settings. The seal is consistent and provides an adequate short term (about 8 hours) seal against solvent evaporation. Because the materials used to form the cap and container are not very rigid, however, the designs cannot provide a seal able to withstand contents under high pressure or provide for long term storage without leakage--even with multiple points of contact. The protrusion on the neck of conventional vials is of increased mass; therefore, dimensional tolerance is not closely controlled during the molding process. Furthermore, the basic snap cap design does not allow for self-aligning or secure retention of the cap and the vial.
Another common closure for containers of this type is a crimp cap, which is securely retained on the neck finish of the container by crimping a metallic (usually aluminum) skirt under a lip on the neck of the container. One advantage of the aluminum crimp cap is that it works on containers having either a standard crimp seal or a snap ring. A disadvantage is that the aluminum crimp cap requires the use of a crimping tool to form a seal. The seal is subject to the amount of squeeze and alignment given by the user. When properly applied, however, the aluminum crimp cap provides a good seal against solvent evaporation.
The crimping tool is made of metal (typically aluminum) to provide the force necessary to deform the aluminum crimp cap and, thereby, either to apply or remove the aluminum crimp cap to or from the container. Removal of an aluminum crimp cap from a container is dangerous. If not done properly, the neck finish of the container can break--leaving ragged glass edges. Moreover, sharp aluminum pieces are exposed as the aluminum crimp cap is literally torn away from the container.
Still another common closure for containers involves a standard screw thread neck finish on the vial and a corresponding screw thread on the cap. Closure is attained and a seal obtained by twisting or rotating the cap onto the vial. Thus, screw thread closures require finger torque pressure to apply and remove the cap. The seal is subject to the amount of torque applied by the user. When torqued properly, the threaded cap provides a good seal equivalent to or better than the aluminum crimp seal. One drawback is that the threaded cap can lose torque upon relaxation of the plastic material, from which the typical threaded cap is made, which allows the cap to back off the threads. In addition, the threaded cap can only be used on threaded vials.
Finally, conventional containers allow only one type of cap per container. Suppliers must maintain large inventories, therefore, of several types of caps and several types of corresponding containers. These containers also are not conducive to simple industrial automation; the only convenient means of handling the containers is with complicated and expensive equipment.